One of the things that has been mentioned on this forum in the past was the CP status downgrade problem. As CP, you have unlimited upgrades, and thus do not get them mailed to you for 10k/20k miles flown. But if you fall to GP or SP the next year, you have no stock of upgrades anymore.
It occurs to me that rather than sending in all of my NAUs now that I am CP, I should save a bunch of those I have recently received, and exchange them this time next year for more with revised expiration dates. Just enough to get me through the first few months.
Still unclear on whether there are NAU for mileage exchange limits - this has been debated in the past.